1. Field of the Invention
The present invention relates to sunglass lenses, particularly to polarized sunglass lenses which enhance contrast and color saturation using narrowband light-filtering means.
2. Description of Related Technology
Recent advances in sunglass lens technology, such as polarized lenses incorporating certain rare-earth compounds, have brought long-desired improvements in perceived color saturation, contrast, and visual acuity. In particular, lenses made in conformance with U.S. Pat. Nos. 6,145,984 and 7,597,441 issued to Farwig demonstrate the advantages of combining light polarization with narrowband light filtration through the use of a glass composition comprising three particular rare-earth oxides in order to achieve a remarkable degree of vision enhancement.
Rare-earth oxides are oxides of metals in the lanthanide and actinide series of the periodic table of elements. When incorporated into optical glass compositions, some of these rare-earth metal oxides favor the transmittance of red, green, and blue primary wavelengths and selectively absorb and reduce the transmittance of certain non-primary wavelengths.
Light filters used in sunglass lenses of the prior art can be divided into two main categories, those which absorb light at certain wavelengths by converting the light energy of those wavelengths into heat energy which is then dissipated within the filter, and those whose front surface has a semi-transparent mirror coating that reflects some of the light back toward its source and away from the filter thereby reducing light transmittance. The majority of prior-art sunglass lenses have comprised light filters which primarily absorb light; some of these have also comprised front-surface mirror coatings which further reduce or modify light transmittance. Lenses with colored semi-transparent mirrors have been used to reduce the transmittance of that portion of the spectrum which corresponds to the color of the mirror.
Polarizers have also been extensively used in sunglass lenses of the prior art. Polarizers reduce excess light (glare) reflected from terrestrial surfaces by exploiting the fact that glare reflected from horizontal surfaces becomes polarized, predominately in a single plane. This means that the electric fields propagated by the reflected light rays share a common planar alignment. When viewing through an optical polarizer, such as a polarized lens, the reflected glare is extinguished if the polarized lens is oriented (rotated) so that its polarization axis opposes by 90 degrees the electric-field plane of the reflected glare. This effect is due to the absorption within the polarizer of the electric field of the reflected glare.
Some light filters used in sunglass lenses of the prior art have comprised a type of light-filtering means referred to herein as “sharp-cut” light-filtering means. This type of light filter is characterized by a steep reduction of transmittance for wavelengths above or below a selected wavelength. Examples of sharp-cut light-filtering means used in sunglass lenses of the prior art include UV filters which block UV wavelengths but transmit visible wavelengths, and IR filters which block IR wavelengths but transmit visible wavelengths.
It has been common practice in the sunglass industry to provide lenses which block ultraviolet (UV) light in the wavelength range from 200-400 nanometers (nm). Sunglass lenses which block all UV and near-UV wavelengths up to 400 nm are given the “UV400” rating. Lenses which block up to 425 nm or 450 nm would thus be rated “UV425” and “UV450” respectively, even though the actual UV spectrum only extends to 380 nm.
It is common belief in the ophthalmic industry that chronic and excess ocular exposure to light in the short-wavelength visible spectrum, i.e., visible violet, can be a factor in the development of a vision-degrading condition known as macular degeneration. Thus it is advantageous for a sunglass lens to block visible violet light as well as UV light. Visible violet wavelengths comprise the range of 380 nm to approximately 450 nm.
Some sunglass lenses of the prior art have provided blocking of all UV light while also blocking all violet and blue wavelengths from 400 nm to as high as 500 nm. Examples of such lenses include the so-called “Blue Blocker” and other similarly-tinted lenses, typically exhibiting a strong orange or amber tint. Lenses of this type are incapable of preserving accurate color perception for the wearer because they remove too much of the less dangerous longer-wavelength portion of the blue spectrum, typically between 460 nm and 500 nm. The result is that blue skies can appear grayish when viewed through such lenses, and practically everything else appears strongly yellowish.
Polarized sunglass lenses of prior art can be prone to fading and discoloration when exposed to chronic, prolonged, and excessive levels of UV light as can occur when the lenses remain in a static position for long periods exposed to direct sunlight. This type of degradation may occur in sunglasses which have been on display in direct sunlight such as in a store window, or frequently left on the dashboard of a vehicle parked in direct sunlight. This is caused by the sensitivity to UV of the color dyes which are usually present in polarizer film along with the iodine that provides polarization. The color dyes give the film and associated lens the desired light transmittance and tint. Many commonly-used organic dyes are prone to UV-induced degradation, an undesirable trait for a sunglass lens. Adhesives used in polarized lenses may also contain dyes that are prone to damage from UV light.
It has been common practice in the manufacturing of prescription polarized sunglass lenses of the prior art to use two lens elements laminated together with a polarizer film encapsulated between the lens elements, the prescription lens element being a clear, untinted composition. This prevents the varying thickness of the prescription lens element from causing a corresponding variation in optical density and light transmittance, i.e., a “vignetting” effect.
In prescription polarized sunglass lenses of the prior art which use a layer of non-UV-blocking tinted glass (including contrast-enhancing types containing rare-earth oxides) and a layer of colorless or nearly-colorless UV-blocking glass, the front lens element is the tinted glass while the rear lens element is the UV-blocking glass. This avoids the aforementioned vignetting effect. Since the vast majority of UV entering a sunglass lens enters from the front, it would be beneficial to incorporate a UV-blocking means in the front lens element of a polarized lens, regardless of whether or not a UV-blocking means is also provided by the rear lens element, in order to protect the polarizer film dyes as previously mentioned.
Sharp-cut light-filtering means used in sunglass lenses of the prior art to selectively block UV wavelengths with minimal effect on visible wavelengths include the incorporation of glass dopants such as cerium oxide, which is known to absorb UV wavelengths below 380 nm; the usage of heat-treated glass compositions containing various copper compounds which block UV; and the usage of plastic compositions containing dyes which selectively absorb UV wavelengths without causing significant absorption of visible wavelengths.
Sharp-cut light-filtering means to block infrared (IR) and near-IR wavelengths have also been used in prior-art sunglass lenses; one such means is the so-called “hot mirror”. This is a front-surface mirror coating which selectively reflects IR and near-IR wavelengths (referred to herein as “deep-red wavelengths”), thereby reducing or blocking transmittance of these wavelengths. Sharp-cut light filters are also known in the optical industry as “square-edge” filters. FIG. 2 depicts the light transmittance of a typical sharp-cut UV light filter, while FIG. 9 depicts the light transmittance of a typical sharp-cut deep-red light filter over the visible range of wavelengths from 400 nm to 800 nm.
It is known that chromatic aberration occurs naturally in the human eye as well as in manufactured optical lenses unless measures are taken to prevent it. Chromatic aberration is the failure of a lens to properly focus all visible wavelengths of light, producing a “fringing” effect visible along the edges of viewed objects. The extreme ends of the visible light spectrum, pure violet and deep-red, comprise the wavelengths that are most difficult for the human eye, as well as for single-element lenses in general, to focus. This is due to the variation in refractive index within a lens that occurs at different wavelengths. Prior-art lenses such as the aforementioned “Blue Blocker” types, which completely block the transmittance of violet, violet-blue, and blue wavelengths greatly reduce chromatic aberration for the wearer, thus improving visual acuity, i.e., sharpness; but these types of lenses have always had a vivid orange-amber tint which degrades color accuracy and perception for the wearer as previously stated.
It would be very beneficial for a polarized sunglass lens to provide enhanced optical contrast, color saturation, and visual acuity while completely blocking UV and visible-violet wavelengths in the front lens element, thus protecting internal color dyes from UV-induced degradation while also protecting the eyes of the wearer from UV and visible-violet light. In the interest of maximum visual acuity for the wearer, it would be of further benefit for such a lens to provide blocking of deep-red wavelengths between 750 nm and 800 nm.